The purpose of hydro-pneumatic pressure accumulators in suspension systems in motor vehicles is to function as a pneumatic spring and accumulate energy and release it when required. For this purpose, a telescopic suspension cylinder normally interacts with the pressure accumulator by means of hydraulic medium. During the deflection movement of the suspension cylinder, the compressible medium, particularly a gas, is compressed by the displaced hydraulic medium within the pressure accumulator, causing the pressure of the compressible medium to be increased. This increased pressure then serves to displace the hydraulic medium and return the suspension cylinder to its original position. Via the suspension cylinder, the subsequently achieved static pressure of the medium creates a lifting capacity that supports the related load.
Various accumulator designs are known. The hydraulic medium can, for example, act directly (without separation wall) against a gas. Accumulators without a separating wall such as this have the advantage that they lack any friction force and that the hysteresis is low. The disadvantage is, however, that during prolonged operating times the gas is absorbed by the pressure fluid. It is therefore practical to separate the hydraulic medium from the compressible medium by means of a displaceable separating wall. The separating wall can be configured as a bladder, a membrane or as a displaceable separating piston.
Membrane or bladder accumulators have the disadvantage that the flexible separating wall (plastic skin), which is particularly comprised of plastic, has the tendency to become brittle at low temperatures and can thus become damaged.
For this reason, piston accumulators of the earlier described type are frequently being used. Such a piston accumulator is disclosed in published application DE 36 13 677 A1, for example. It is an integrated embodiment, wherein the separating piston is arranged inside the hollow piston rod of a suspension cylinder. The accumulator housing is therefore in this case formed by the hollow piston rod.
A similar embodiment is also described in DE 38 39 446 A1.
An additional hydro-pneumatic piston accumulator is also described in DE 90 02 419 U1. It is a particularly specialized embodiment as a pressure converter, wherein the separating piston is designed with two differently sized effective pressure areas.
Piston accumulators of the generic type have the disadvantage that the piston must be relatively long in order to be guided inside of the accumulator housing without tilting. More specifically, the length of the separating piston must at least be equal to its approximate diameter. If the length of the separating piston is significantly less than its diameter, then the separating piston may tilt and stick. This effect is known as a “stick-slip-effect.” The relatively great length of the separating piston can in particular be clearly seen in the item of DE 36 13 677 A1.
The object of the present invention is to improve a piston accumulator of the aforementioned type in such a manner that it can be constructed having a short and compact length, yet will still ensure positive use characteristics.
According to the invention, the advantages are achieved claim 1 in that the separating piston with its interior guide opening is guided on an elongated guide element, which is axially fixed inside the accumulator housing. The guide element is advantageously designed with a circular cross section and is positioned centered relative to the movement axis of the separating piston. Since the guide element has a small diameter, which is significantly smaller than the outer diameter of the separating piston, the effective guide length of the interior guide opening can also be significantly shorter. In this case as well, the guide length is approximately equal to the diameter of the guide opening and/or that of the guide element.
According to the invention, the separating piston is therefore dually guided inside the accumulator housing, first in its outer circumferential area, in that it has an outer circumferential gasket to seal a circumferential annular gap from a cylindrical interior wall of the accumulator housing, and second, in the area of the interior guide opening by means of the rod-shaped guide element. It is preferable that the external guide and the interior guide according to the invention are axially offset in relation to each other. This is preferably achieved by positioning the guide opening in the area of an axially protruding, hollow cylindrical guide abutment, said abutment being comprised of the relatively flat disc-shaped separating piston. In another preferable arrangement, the guide opening can be formed by a guide bushing, which is positioned inside the guide abutment and which encloses the guide element.
Further preferable arrangement characteristics of the invention are incorporated in the dependent claims as well as in the following description.